1. Field of the Invention
This invention relates to a tractor transmission having a change speed mechanism, a shift lever for shifting the change speed mechanism, a super-reduction mechanism having a larger reduction ratio than the change speed mechanism for decelerating and outputting drive inputted thereto, and an overdrive mechanism for accelerating and outputting drive inputted thereto.
2. Description of the Related Art
A typical tractor transmission known in the art, as disclosed in Japanese Patent Laying-Open Publication H10-287145 for example, has separate control devices for controlling the super-reduction mechanism and overdrive mechanism. The increased number of control devices adds to the difficulty in securing an installation space. It is also necessary to provide a check mechanism for disabling a simultaneous operation of the two control devices. This check mechanism is complicated and causes cost increases.
In the tractor with the transmission having the super-reduction mechanism and overdrive mechanism besides the ordinary change speed mechanism, the super-reduction mechanism and overdrive mechanism are selectively used according to operating conditions. Of course, the number of speeds finally produced is the same whether the super-reduction mechanism or overdrive mechanism is used or not.
However, an optimal number of speeds is variable with operating conditions. It is desirable to vary the number of speeds finally provided by the use of the super-reduction mechanism or overdrive mechanism according to operating conditions.
An object of this invention to provide a simplified control system for a super-reduction mechanism and an overdrive mechanism. It is another object of the invention to improve the work efficiency of a tractor with a transmission having a super-reduction mechanism and an overdrive mechanism.
The above object is fulfilled, according to this invention, by a tractor transmission comprising a change speed mechanism, a shift lever for shifting the change speed mechanism, a super-reduction mechanism for decelerating and outputting power inputted thereto, the super-reduction mechanism having a larger reduction ratio than the change speed mechanism, an overdrive mechanism for accelerating and outputting power inputted thereto, and a single switching device for operating both the super-reduction mechanism and the overdrive mechanism, the switching device is shiftable, when the super-reduction mechanism is in neutral, from a state of operating the super-reduction mechanism to a state of operating the overdrive mechanism.
In this construction, the super-reduction mechanism and overdrive mechanism are operable by the single switching device, thereby simplifying the control device. Only when the super-reduction mechanism is in neutral, the switching device is shiftable from the state of operating the super-reduction mechanism to the state of operating the overdrive mechanism. The super-reduction mechanism must be placed in neutral whenever overdrive controls are effected. Thus, no check mechanism is required for disabling a simultaneous operation of the two mechanisms.
As a preferred embodiment for specifically achieving the above features, a construction is proposed wherein the switching device has a shifting region including a super-reduction control region for operating the super-reduction mechanism, an overdrive control region for operating the overdrive mechanism, and a bridge region bridging the super-reduction control region and the overdrive control region, the super-reduction mechanism being in neutral when the switching device is located at a point in the super-reduction control region for shifting to the bridge region. To give the driver a clear indication as to shifting between the super-reduction control region and overdrive control region, the bridge region preferably extends substantially perpendicular to the super-reduction control region and the overdrive control region.
In a preferred embodiment of this invention, an overdrive detecting sensor is provided for detecting the switching device having shifted from the bridge region to the overdrive control region. This construction enables a prompt control operation which must be carried out in using the overdrive mechanism.
In a further preferred embodiment, a speed display surface is provided for displaying speed stages of the overdrive mechanism as well as speed stages of the change speed mechanism. In this construction, when the overdrive mechanism is used and the shift lever is operated for shifting the change speed mechanism, the driver may confirm the speed stages when using the overdrive mechanism, from the speed display surface provided for displaying the speed stages of the overdrive mechanism. This improves operability of the shift lever when the overdrive mechanism is used.
To improve the work efficiency of a tractor with a transmission having a super-reduction mechanism and an overdrive mechanism, this invention proposes a tractor transmission comprising a change speed mechanism, a shift lever for shifting the change speed mechanism, a super-reduction mechanism for decelerating and outputting power inputted thereto, the super-reduction mechanism having a larger reduction ratio than the change speed mechanism, an overdrive mechanism for accelerating and outputting power inputted thereto, and a switching device for switching the overdrive mechanism, wherein the change speed mechanism has a speed stage determined by a shift position of the shift lever, the speed stage being variable with a control position of the switching device.
This construction provides a different final number of speed stages when the switching device is operated to vary a running state, i.e. when the overdrive mechanism is used. Thus, a large number of speeds are made available for a field operation, i.e. when the overdrive mechanism is unused. When moving at high speed, e.g. from one field to another, by using the overdrive mechanism, a reduced number of speeds are provided, a large number of speeds being unnecessary.
Other features and advantages of this invention will be apparent from the following description of the embodiment to be taken with reference to the drawings.